IMPROVEMENTS IN OR RELATING TO FISHING APPARATUS

Abstract: A hydrodynamic float element (1) for use with apparatus for "Trawl" fishing by attachment to the head-rope of a trawling net, consists of a body having a leading surface (5) for producing lift during towing and stabilising means, which body has a leading portion (2) providing the greater part of the buoyancy of the element wherein the said body further comprises a trailing portion (3) which is generally flattened and thinner than the leading portion and said trailing portion of the body has a stabilising means (4) extending upwardly therefrom to provide passive directional stability to the float element in use and additional hydrodynamic lift.

Description: "IMPROVEMENTS IN OR RELATING TO FISHING APPARATUS" This invention relates to fishing apparatus and, more specifically, to a hydrodynamic float for use with apparatus for "Trawl" fishing. Trawl fishing is well-known and the apparatus for such fishing comprises a bag-like net with an open mouth which is drawn through the water at a predetermined speed and depth whereupon fish in the path of the open mouth of the net are entrained and captured therein. The mouth of the trawl net is held open at each side by so-called "otter-boards", which are inclined to the direction of travel through the water and whereby, when the trawl net is being towed, water flows over the otter-boards to force the sides of the mouth of the net apart. The lower regions of the mouth of the trawl, defined by a so-called "foot-rope", are weighted by a plurality of weights spaced apart along the said foo -rope and the upper regions of the mouth, defined by a so-called "head-rope", have buoyancy members spaced apart along said head-rope. A conventional deep sea trawling net may have up to, and often in excess of, one hundred and fifty buoyancy elements along the head-rope and, conventionally, some ten to fifteen of such buoyancy elements are spaced apart along a so-called "bog-rope", and the bog-rope is lashed to the head-rope of the trawling net at locations between the float elements. A head-rope may thus have some 10-20 bog-ropes attached end to end therealong. The present invention is concerned with the buoyancy elements for maintaining the head-rope elevated with respect to the foot-rope. The buoyancy elements currently in use are of generally spherical form, constructed of wood, hollow glass or hollow fibre glass. Such buoyancy elements may, when the buoyancy element is made from a solid material, have a hole bored therethrough to allow the buoyancy elements to be threaded onto the bog-rope. When said buoyancy elements are of hollow construction they may have a hole therethrough defined by a tubular element, passing through the element and sealed at its ends to the spherical body, thereby maintaining the air volume within the spherical body whilst allowing the buoyancy elements to be threaded onto a bog-rope. In an alternative arrangement the buoyancy members may be captive in net-like bags and the said bags are secured to the bog-ropes. A serious disadvantage of the conventional buoyancy elements for the head-rope of a trawling net arises from the fact that the upward force exerted upon the head-rope is solely dependent upon the buoyancy of said elements, which is limited. Due to the very high stresses in the head-rope, imparted by the otter-boards and which stresses increase with the speed of towing through the water, at high towing speeds, the floatation elements have such a limited effect on the head-rope that the head-rope can sag and partially close the mouth of the trawl net. There have been proposals to provide special floats intended to address such problems. One such proposal is described in GB-A-399 701 where a float is provided in the form of a streamlined body having stabilising vertical and horizontal tail fins. The float has further laterally extending side fins whose purpose is to act as a kite and provide lift additional to the natural buoyancy of the float. That float is a torpedo shaped hollow aluminium body where the centre of buoyancy lies ahead of the centre of gravity in the intended direction of towing. This is intended to counter any tendency of the float to dive rather than lift. In order to compensate for the pressures of deep sea trawling, the body of the float is filled with gas at superatmospheric pressure. From the foregoing it will be recognised that the construction of the float although not complex is more complicated than one might wish for and accordingly its expense is a significant consideration in comparison with the traditional spherical floats. This proposal was made some sixty years ago and yet there is no evidence that it has been found commercially useful since spherical floats are still widely used in the fishing industry. An alternative structure was proposed about forty years ago in GB-682 349 where a lifting device or kite is described. This has a curved buoyant lifting member formed from a hollow metal sheath to which are attached transversely-disposed hollow cylinders to provide strength and improve buoy?icy. Tubular members are fastened to the cylinders to provide support for a deck and rudder made from ribbed metal sheeting. The arrangement once again is relatively complex and does not appear to have found widespread interest. An object of this invention is to provide a float element which obviates or mitigates the aforesaid disadvantages. It is considered that the device to described hereinbelow is relatively cheap to manufacture, is made of the minimum number of components, offers improved strength and durability, and increased reliability and performance over the devices in current widespread use . Particularly, the present invention seeks to provide a float element for the head-rope of a trawling net which element is not totally dependent upon its natural buoyancy for imparting an upward force on the head-rope. According to the present invention there is provided a float element for the head-rope of a trawling net comprising a body having at least one leading surface for producing hydrodynamic lift during towing and stabilising means, which body has a leading portion providing the greater part of the buoyancy of the element characterised in that, the said body further comprises a trailing portion which is generally flattened and thinner than the leading portion and said trailing portion of the body has stabilising means extending upwardly therefrom to provide passive directional stability to the float element in use and additional hydrodynamic lift. Preferably, the body of said element has a length greater than its breadth, hydrodynamic lift being provided at least in part by virtue of said leading portion having an upper leading edge and a lower leading edge with a leading surface therebetween which is inclined, with respect to a notional horizontal plane, at an angle in the range of from 35° to 60° , preferably in the range of from 45° to 55°. Preferably also the element has a body configured to present a generally planar front surface which is inclined to the central axis of the float element so that the upper regions of said front surface are forward of the lower regions thereof when the trawling net is being towed. However said front surface may be curved slightly without detracting from its ability to produce hydrodynamic lift. A preferred structure of the float element is characterised by an essentially solid body which has a leading portion providing the greater part of the buoyancy of the element, said portion having a upper leading edge and a lower leading edge with a leading surface therebetween which is inclined, with respect to a notional horizontal plane, at an angle in the range of from 35° to 60°, preferably in the range of from 45° to 55°, the said body further comprising a trailing portion which is generally flattened and thinner than the leading portion and said trailing portion of the body further has stabilising means extending upwardly therefrom to provide passive directional stability to the float element in use and additional hydrodynamic lift . Preferably also the trailing portion has a generally planar undersurface terminating in a downturned lip which assists in countering a tendency for the element to flip over backwards from front to rear due to lift created by water pressure on the front surface (s) thereof during towing. The trailing portion may be provided with means for fastening it to the mesh of the trawl net in order to ensure that the float adopts the correct attitude upon first contact with the water without becoming entrapped under the net when the net is being deployed from the trawler. The undersurface of the body of the improved float element to be described more particularly hereinbelow may be recessed or of generally concave configuration. The opposing longitudinal sides of the said body should be smooth surfaces which preferably diverge with respect to the undersurface from which they extend, and notwithstanding said divergence, may each present a parabolic surface which is preferably convex with respect to the opposite side. The longitudinal side surfaces which function to contribute significantly to hydrodynamic lift preferably form at least in part upstanding fins to thereby also provide passive directional stability as the element is being towed through the water. Preferably the angle between the front surface and the bottom surface is greater than 110°. Preferably the float element presents a substantially planar surface on each side and said side planar surfaces make an angle of about 55° to the horizontal. Preferably the float element is provided with fastening means for attachment to the trawl net which permits swivelling of the element to allow it to continually head into the direction of trawling and thereby maximise the upthrust resulting from the water flowing over the leading portion of the body thereof. The float element is conveniently moulded from plastics material and the material selected for construction of the float element must obviously be of a density providing sufficient buoyancy in water. The element may be a plastics shell filled with a substantially rigid foam plastics material but preferably is moulded in a single step from one plastics material such as a foamed polyolefin e.g. foamed polyethylene or polypropylene. This essentially solid construction avoids implosion risks when the element is used for deep trawling as observed with the prior art type. The invention will now be described further by way of example with reference to the accompanying drawings in which: Fig. 1 shows a left side view of a float element of the invention; Fig. 2 shows a front view of the float element shown in Fig 1; and Fig. 3 shows a view of the underside of the float element shown in Figs . 1 and 2. In the embodiment illustrated in the drawings a float element 1 has a leading head portion 2, a trailing tail portion 3 and longitudinal sides forming divergent fins 4. The head portion presents a planar front surface 5 having an upper leading edge 6 and side edges 7 converging therefrom towards a lower leading edge 17, the said front surface being forwardly inclined with respect to a notional horizontal plane about a front to rear central axis CA. The upper surface of the float element has a profile which initially" ~ rises from the leading edge 6 at a shallow angle to provide a crown portion 8 before sweeping abruptly downwards between the side fins 4 and levelling out towards the tail portion 3. In this way there is a trough 9 formed between the side fins 4. The head portion provides hydrodynamic lift by virtue of the inclined front and side surfaces and also provides buoyancy means by virtue of the selected material of construction. The underside of the element comprises a substantially planar bottom surface 10 having side edges 11 diverging towards the tail portion 3. The tail portion itself comprises a downturned lip 13. At least one aperture 14 is provided in the tail portion to permit the tail to be tied to the mesh of a net which enables the element to enter the water with the correct attitude without becoming possibly entrapped under the net. In the illustrated embodiment the planar front surface 5 and the planar bottom surface 10 are configured and arranged as opposed substantially triangular surfaces meeting at lower leading edge 17. Said front and bottom surfaces 5, 10 with lower outboard portions of the side fins 4 to provide a lower surface configuration of an essentially pyramidal configuration. Ea... side fin 4 has apertures 15 in the region of the adjoining apexes of the front and bottom surfaces providing access to a sealed transverse throughbore 16 for the purposes of attaching the float element to a trawl net . The planar front surface is inclined some 60° to the vertical and the bottom surface 10 makes an angle of some 123° with the front surface 5. This angle may vary between 110 and 140 degrees. The two side fins 4 make an angle of some 55° to a notional horizontal plane parallel with the bottom surface 10 and this angle may vary between 40° and 70°. In use the element is attached to a bog-rope e.g. by simply passing a tie-line through the throughbore 16. The manner of attachment may optionally include a swivel. The throughbore 16 may be lined with an abrasion resisting material. A plurality of float elements may be threaded onto a single bog-rope and the bog-rope is attached to a head-rope of a trawl net, as by lashings, at suitable spaced intervals along the length of the bog-rope. The head-rope will have the upper regions of the trawl net attached thereto. The tail portion of each element may be tied to the mesh of the net to ensure correct deployment without entanglement in the net. Normally, when the trawl net is being towed through the water the towing lines are attached to the otter-boards and there is a natural tendency for the head-rope to drag in its mid-regions so that, in plan view, the head-rope adopts an arcuate configuration. When a float element of this invention is in a mid- region of the head-rope, so that the central axis CA lies substantially parallel to the direction of displacement through the water, the pressure of the water on the front surface of the element will tend to cause the element to rotate about the bog-rope. This rotation is resisted by the water flowing over the upper and lower tail surfaces, the latter particularly providing a countering effect to such rotation. Directional stability of the element is maintained by virtue of the water flow over both sides of the side fins 4 i.e. both through the trough 9 and around the sides of the element. The element 1 will therefore adopt a predicted relatively stable position with respect to the net during towing through the water such that the front surface will remain inclined to the water. This predetermined attitude is maintained during towing due to the design of the element . The combination of natural buoyancy and water flow over the inclined front and upstanding side surfaces generates a hydrodynamic lift which will be communicated to the bog-rope and, therefrom, to the head-rope to urge the head-rope upwardly. The embodiment just described is manufactured entirely from polyethylene which is foamed but it could be equally manufactured from a glass reinforced resin (GRP) shell filled with polyurethane. It may be manufactured from plastics materials by suitable moulding methods such as injection-moulding, blow-moulding or rotational moulding. Other forming operations are not excluded. Likewise, use of natural materials such as wood or rubber, or other formable materials such as metal or concrete are not excluded. It will be seen that with the float element proposed by the invention, which is capable of imparting an upward force on the head-rope greater than the natural buoyancy force alone, two important advantages can be obtained. Firstly, because of the increased hydrodynamic lift generated by each float element, fewer float elements are required to maintain the head-rope elevated and, secondly, because the upward force generated by each float element will increase with increase in the speed through the water, higher towing speeds are possible. Furthermore, it will be seen that the improved float element proposed herein may be readily manufactured by moulding in plastics, it has an improved shape increasing lift and buoyancy whilst minimising drag to improve towing efficiency and by providing an essentially solid construction in contrast to the known prior art it is proof against implosion and may be used reliably at significantly greater depths. Whilst the present invention has been described by way of example with reference to a specific embodiment the invention is not restricted thereto and many modifications and variations will be apparent to persons skilled in the art.

Claims: Claims 1. A float element (1) for the head-rope of a trawling net comprising a body having a leading surface (5) for producing lift during towing and stabilising means, which body has a leading portion (2) providing the greater part of the buoyancy of the element characterised in that, the said body further comprises a trailing portion (3) which is generally flattened and thinner than the leading portion and said trailing portion of the body has stabilising means (4) extending upwardly therefrom to provide passive directional stability to the float element in use and additional hydrodynamic lift. 2. A float element according to claim 1 wherein the body of said element has a length greater than its breadth, hydrodynamic lift being provided at least in part by virtue of said leading portion having an upper leading edge (6) and a lower leading edge (17) with a leading surface (5) therebetween which is inclined, with respect to a notional horizontal plane, at an angle in the range of from 35° to 60° . 3. A float element according to claim 2 wherein the leading surface (5) is inclined at an angle in the range of from 45° to 55° . 4. A float element according to any one of the preceding claims wherein the body of the float element has a planar front surface (5) having an upper leading edge (6) and side edges (7) converging therefrom towards the underside of the body, the said front surface being forwardly inclined with respect to a notional horizontal plane about a front to rear central axis CA, said body having an upper surface having a profile which initially rises from the leading edge at a shallow angle to provide a crown portion (8) before sweeping abruptly downwards and levelling out to provide a generally flattened upper surface for the tail portion (3) , said body further having longitudinal side surfaces which function to contribute significantly to hydrodynamic lift and form at least in part divergent side fins (4) extending rearwardly from the head portion and upwardly from the tail portion, which also act to provide hydrodynamic lift, to form a trough (9) defined between the side fins and the upper surface of the tail portion, and the underside of the body comprises a substantially planar bottom surface (10) having side edges (11) diverging towards the tail portion. 5. A float element according to claim 4 wherein the planar front surface (5) and the planar bottom surface (10) are configured and arranged as opposed triangular surfaces, and at the adjoining apexes there is provided an eyelet for attaching means for securing the float element to a trawl net . 6. A float element according to claim 5 wherein the planar front surface is inclined about 60° to the vertical and the bottom surface makes an angle of about 120° with the front surface. 7. A float element according to claim 5 wherein the divergent side fins (4) are inclined to a notional horizontal plane at an angle of about 55°. 8. A float element according to claim 4 wherein the tail portion has at least one aperture (14) for use in fastening the tail portion to the mesh of a trawl net. 9. A float element according to any preceding claim wherein the tail portion comprises a downturned lip (13) formed at the rearmost edge thereof . 10. A float element according to any one of the preceding claims wherein the body is essentially formed entirely from a plastics material which may be a foamed polyolefin. 11 A float element substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

Inventors: DIXON JAMES LAWRENCE, DIXON, JAMES, LAWRENCE
Applicants: DIXON JAMES LAWRENCE, DIXON, JAMES, LAWRENCE
Application year: 1994
Main classification: A01K73
IPC codes: A01K73